Chemistry Reference
In-Depth Information
Fig. 25 Schematic
mechanism of the
E
,
Z
-
isomerization with
simultaneous inversion of
one moiety of the
anti
-
folded conformation a-
C
2h
(
y
) via an orthogonally
twisted/folded transition
State ft
⊥
-
C
s
(
d
)
1'
1'
1
1
a
1'
E-RS'
1
ft
┴
PR
ft
┴
PS'
1'
1
1'
1'
┴
MR
1
1'
ft
ft
┴
MS'
1
1
a
Z-RR'
a
Z-SS'
1'
1
1'
┴
PS
ft
1
ft
┴
PR'
1'
1
1'
1'
1
1
ft
a
ft
┴
MS
┴
MR'
E-SR'
For the conformations a-
C
2h
(
y
), ta-
C
2
(
y
), f-
C
s
(
xz
), and a-
C
i
, symmetry
constraints do not allow
E
,
Z
-isomerization. The plane
(
xz
)ina-
C
2h
(
y
) and f-
C
s
(
xz
) reflects the left half of each moiety onto the right half. The two halves cannot
change sides without coinciding in the plane or breaking the symmetry. Likewise,
the center of symmetry
i
in a-
C
i
relates atoms of different moieties which have a
trans
relationship. An
E
,
Z
-isomerization would necessarily break this
trans
rela-
tionship and the
C
i
symmetry. In ta-
C
2
(
y
), the
C
2
symmetry axis interrelates atoms
cis
with respect to the central double bond. They would be
trans
after an
E
,
Z
-
isomerization. Thus an
E
,
Z
-isomerization cannot be accomplished within the con-
formational subspaces of
C
2h
(
y
),
C
2
(
y
),
C
s
(
xz
), or
C
i
symmetry. The conforma-
tions a-
C
2h
(
y
), ta-
C
2
(
y
), f-
C
s
(
xz
), and a-
C
i
may thus be excluded as transition
states for the
E
,
Z
-isomerization. See, however, Sect.
4.3.7
for a conformational
isomerization of the
anti
-folded and
syn
-folded conformations with simultaneous
E
,
Z
-isomerization conserving
C
2
symmetry.
The only possible conformations for transition states of a single step
E
,
Z
-
isomerization with simultaneous inversion of one moiety without intermediates or
bifurcations have low symmetry: ft
⊥
-
C
s
(
d
)orft
⊥
-
C
s
(
d
0
) with one planar moiety
perpendicular to the second, symmetrically folded moiety, or ft-
C
1
, a transition
state (and pathway) without any symmetry.
The transition states ft
⊥
-
C
s
(
d
) and ft
⊥
-
C
s
(
d
0
) have a point group order
h
TS
¼
˃
2;
thus, there are eight versions of this conformation. The mechanism for this process
with connectivity
C
¼
2 and
p
¼
2 parallel pathways is schematically outlined in
Fig.
25
.
Along the pathway from a
Z-RR
0
to a
E-RS
0
, the folding of the first moiety remains
more or less constant, while the second moiety unfolds and rotates. At the transition
state, the second moiety is planar and bisecting the first moiety. Folding in the
opposite direction and continued rotation leads to the product. Interchanging the
roles of the two moieties, i.e., inverting and rotating the first moiety while keeping
the folding of the second moiety constant, leads to a different product, a
E-SR
0
(hence
